This invention relates to projection lenses and, in particular, to compact, telecentric projection lenses having large effective back focal length to focal length ratios for use in forming an image of an object composed of pixels, such as, an LCD, a reflective LCD, a DMD, or the like.
As used in this specification and in the claims, the following terms shall have the following meanings:
(1) Telecentric
Telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity. Since light can propagate through a lens in either direction, the pupil at infinity can serve as either an entrance or an exit pupil depending upon the lens"" orientation with respect to the object and the image. Accordingly, the term xe2x80x9ctelecentric pupilxe2x80x9d will be used herein to describe the lens"" pupil at infinity, whether that pupil is functioning as an entrance or an exit pupil.
In practical applications, the telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens"" optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms xe2x80x9ctelecentricxe2x80x9d and xe2x80x9ctelecentric lensxe2x80x9d are intended to include lenses which have at least one pupil at a long distance from the lens"" elements, and the term xe2x80x9ctelecentric pupilxe2x80x9d is used to describe such a pupil at a long distance from the lens"" elements. For the projection lenses of the invention, the telecentric pupil distance will in general be at least about 10 times the lens"" focal length.
(2) Effective Back Focal Length
The effective back focal length (BFL) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the back surface of the rearward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
(3) Forward Vertex Distance
The forward vertex distance (FVD) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the front surface of the forward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
Projection lens systems (also referred to herein as xe2x80x9cprojection systemsxe2x80x9d) are used to form an image of an object on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
The basic structure of such a system is shown in FIG. 5, where 10 is a light source (e.g., a metal halide or a high pressure mercury vapor lamp), 12 is illumination optics which forms an image of the light source (the xe2x80x9coutputxe2x80x9d of the illumination system), 14 is the object which is to be projected (e.g., an LCD matrix of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16.
For front projection systems, the viewer will be on the left side of screen 16 in FIG. 5, while for rear projection systems, the viewer will be on the right side of the screen. For rear projection systems which are to be housed in a single cabinet, a mirror is often used to fold the optical path and thus reduce the system""s overall size.
Projection lens systems in which the object is a pixelized panel are used in a variety of applications. Such systems preferably employ a single projection lens which forms an image of either a single panel having red, green, and blue pixels or of three panels, one for red light, a second for green light, and a third for blue light. In some cases, e.g., large image rear projection systems, multiple panels and multiple projection lenses are used, with each panel/projection lens combination producing a portion of the overall image. In either case, projection lenses used with such systems generally need to have a long effective back focal length to accommodate the prisms, beam splitters, color wheels, etc. normally used with pixelized panels.
A particularly important application of projection lens systems employing pixelized panels is in the area of microdisplays, e.g., front projection systems which are used to display data and rear projection systems which are used as computer monitors. Recent breakthroughs in manufacturing technology has led to a rise in popularity of microdisplays employing digital light valve devices such as DMDs, reflective LCDs, and the like.
Projection displays based on these devices offer advantages of small size and light weight. As a result, a whole new class of ultra portable lightweight projectors operating in front-projection mode and employing digital light valves has appeared on the market. Lightweight compact rear projection systems can also be achieved through the use of these devices.
To display images having a high information content, these devices must have a large number of pixels. Since the devices themselves are small, the individual pixels are small, a typical pixel size ranging from 17xcexc for DMD displays to approximately 8xcexc or even less for reflective LCDs. This means that the projection lenses used in these systems must have a very high level of correction of aberrations. Of particular importance is the correction of chromatic aberrations and distortion.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field.
All of the aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, astigmatism, and distortion typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction, including correction of secondary lateral color, is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
The above-mentioned microdisplays typically require that the light beam from the illumination system has a near-normal angle of incidence upon the display. In terms of the projection lens, this translates into a requirement that the lens has a telecentric entrance pupil, i.e., the projection lens must be telecentric in the direction of its short imaging conjugate where the object (pixelized panel) is located. This makes the lens asymmetric about the stop which makes the correction of lateral color more difficult.
In addition to the foregoing, for rear projection systems, there is an ever increasing demand for smaller cabinet sizes (smaller footprints). In terms of the projection lens, this translates into a requirement that the lens has a wide field of view in the direction of the image (screen). This requirement makes it even more difficult to correct the lateral color of the lens. Similarly, the requirement for a long effective back focal length also makes it more difficult to correct lateral color.
In addition to having a wide field of view, the demand for smaller cabinet sizes also translates into a requirement that the lens has a short forward vertex distance. In this way, the amount of linear space that must be allocated to the projection lens in the cabinet is reduced. Also, for a given field of view, a shorter forward vertex distance results in a smaller maximum clear aperture for the lens elements used in the projection lens. This not only reduces the maximum transverse dimension of the lens, but also reduces its weight and cost. As with effective back focal length, reducing the forward vertex distance makes it more difficult to correct the aberrations of the lens. In particular, a shorter forward vertex distance generally requires stronger lens elements whose aberrations are more difficult to correct.
Achieving a short focal length, a long effective back focal length, a wide field of view in the direction of the lens"" long conjugate, and a short forward vertex distance simultaneously, while still maintaining the high level of aberration correction needed for a projection lens system which employs pixelized panels is particularly challenging since these various requirements tend to work against one another. As discussed and illustrated below, the present invention provides projection lenses which satisfy these conflicting criteria.
In view of the foregoing, there exists a need in the art for projection lenses for use with pixelized panels which have some and preferably all of the following properties:
(1) a high level of lateral color correction, including correction of secondary lateral color;
(2) low distortion;
(3) a large field of view in the direction of the image;
(4) a telecentric entrance pupil;
(5) a long effective back focal length;
(6) a short forward vertex distance; and
(7) a small maximum clear aperture for the lenses making up the projection lens.
To satisfy this need in the art, the invention provides projection lenses which have some and preferably all of the above seven features.
In particular, the invention provides a projection lens for forming an image of a pixelized panel, wherein the projection lens has a long conjugate side (image or screen side) and a short conjugate side (object or pixelized panel side) and comprises in order from its long conjugate to its short conjugate of:
(A) a first lens unit (U1) having a negative power and comprising at least one negative lens element (N1) of overall meniscus shape, said negative lens element being convex towards the long conjugate side and comprising at least one aspheric surface; and
(B) a second lens unit (U2) having a positive power, said second lens unit being separated from the first lens unit by an axial space and comprising at least one positive lens element (P1) which comprises at least one aspheric surface;
wherein:
(i) the projection lens is telecentric on the short conjugate side; and
(ii) the projection lens has an effective focal length f0, an effective back focal length BFL, and a forward vertex distance FVD which satisfy the following relationships:
BFL/f0 greater than 3.5; and 
FVD/f0  less than 20.
Preferably, the BFL/f0 ratio is greater than 4.0 and in some cases can be greater than 4.5 and even greater than 5.0. Similarly, the FVD/fo ratio is preferably less than 17 and in some cases can be less than 15 and even less than 13.
In addition to having the above BFL/foand FVD/f 0 ratios, the projection lenses of the invention preferably have a field of view xcex8 in the direction of the long conjugate of at least 70xc2x0 (e.g., 75xc2x0xe2x89xa6xcex8xe2x89xa680xc2x0) and a maximum clear aperture to f0 ratio (D/f0 ratio) which is less than 5.0 and in some cases is less than 4.0.